Ponder:
How can you tell by sight whether your lens is converging or diverging? How can you tell by touch?
Converging Lens:
Since objects are imaged on the focal plane, use your lens to form the sharpest image of the sun on a piece of paper, as you would with a burning glass. Measure the distance from the lens to the paper to obtain the focal length. Suppose the distance is 62.5 cm = 0.625 m. The power of the lens, and therefore its prescription is then 1/0.625 = 1.6 diopters.
Diverging Lens:
Find a magnifying glass and measure its focal length by the technique described above. Place it next to your eyeglass lens and measure the focal length, and hence the power of the combination. (If it is not converging, find a stronger magnifying glass.)The power of your lens is the power of the combination minus that of the magnifying glass alone. Suppose the combination has a power of 1.0 diopters (f=1 m) and the magnifying glass has a power of 4.0 diopters (f=1/4 m). Your lens power is then -3.0 diopters.
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